Experimental Cosmic Microwave Background

Draft version February 15, 2010 Preprint typeset using LATEX style emulateapj v. 08/13/06

EXPERIMENTAL COSMIC MICROWAVE BACKGROUND RESEARCH IN CANADA WHITEPAPER FOR THE CANADIAN DECADAL LRP Matt Dobbs1, Dick Bond2,3, Mark Halpern4, Gil Holder1, Barth Netterfield2, Douglas Scott4 Draft version February 15, 2010

ABSTRACT The cosmic microwave background (CMB) is a cornerstone for precision cosmology and Canadian experimeters have been involved with the majority of signiﬁcant results over the last decade. In this whitepaper, highlights from this period are presented along with a summary of our unique capabilities. With the temperature anisotropies accurately characterized out to moderate angular scales (` < 700) the community is focused on (1) small angular scale temperature anisotropy measurements to study the intervening matter through CMB secondaries and (2) the CMB polarization onto which the signature of inﬂationary gravity waves may be imprinted, allowing us to probe back to a fraction of a second after the big bang. In parallel with this, CMB experimenters are also planning new projects at other wavelengths, continuing the already established tradition (e.g. BLAST and SCUBA2) of using expertise and technology originally developed for mm-wave CMB observations to break new ground elsewhere. Subject headings: Cosmic Microwave Background, Cosmology, Canada

1. INTRODUCTION At small angular scales there is a wealth of information Measurements of the Cosmic Microwave Background about inflation and the growth of cosmic structure. The radiation have provided a cornerstone to our understand- tilt of the primordial power spectrum is a key parameter ing of cosmology, anchoring theories with precision mea- encoding information about inflation. Beyond the expo- surements of the universe at z=1100, about 380,000 years nentially damped tail, sources of secondary anisotropy after the Big Bang. The CMB alone provides tremendous dominate the power spectrum. The signature imprinted resolving power for the cosmological parameters and it is by inverse-Compton scattering of CMB photons with hot rare to report any parameter measurement without using electrons in the intra-cluster medium, called the Sunyaev CMB data to constrain the fit. Canadian scientists and Zel’dovich (SZ) effect, provides a means of identifying technology have been involved in nearly every important distant galaxy clusters. The surface brightness of the CMB result over the last decade and these measurements SZ effect is insensitive to redshift and closely related to have established the standard cosmological model. cluster mass, making it an ideal selection function for As the Canadian community enters into the process of using the growth of structure through cluster counts to its Long Range Plan, the WMAP collaboration, with in- constrain cosmology. The small scale anisotropies are volvement from CITA and UBC, has published its 7 year affected by weak lensing, providing a measure of the in- results (Jarosik et al. 2010, e.g.) and mapped the CMB tegrated matter along the line of sight which, amongst temperature anisotropies to the cosmic variance limit out other things, can be used to weigh the neutrino species. to ` ∼ 550. By many measures, the WMAP papers are 1.2. CMB Polarization the most cited scientific results in history. The Planck Satellite has been successfully launched and produced its Experiments are only just beginning to have the sensi- First Light survey 6. It is now in a mode of routine ob- tivity necessary to map the faint CMB polarization. The servations. science is tantalizing. While the microwave background With definitive measurements of the temperature provides a data point 380,000 years after the big bang, anisotropy out to moderate angular scales, the CMB gravity waves emitted from an early period of inflation community began refocusing its efforts about 5 years ago carry information from a tiny fraction of a second after on fine angular scale anisotropies (` > 2000) and polar- the big bang. If inflation occurred at a high enough en- ization. In both cases, more capable experiments have ergy scale, as the simplest models suggest, inflationary had to wait for technological breakthroughs–many with gravity waves will imprint a measurable large angular Canadians playing key roles. While early results have scale signature on the CMB polarization. One compo- been reported, definitive measurements will require the nent of this signature, the “B-mode” polarization, pro- better part of the next decade or longer. vides a clean imprint on the microwave background. This is an experimental probe of the highest energy scales and 1.1. Fine Scale CMB Anisotropy earliest time in cosmic history. At small angular scales, secondary scattering with matter along the line of sight provides a weak lensing signa- 1 McGill University ture that is measurable in the B-mode power spectra. 2 University of Toronto 3 Canadian Institute for Theoretical Astrophysics Much of the effort in the international CMB commu- 4 University of British Columbia 5 Author names with (?) have not stated explicitly that they are nity today is focused around galaxy cluster surveys using willing to co-author. fine angular scale CMB anisotropies and making defini- 6 http://www.esa.int/esaCP/SEM5CMFWNZF index 0.html tive measurements of the CMB B-mode power spectrum. 2

2. CANADIAN ACHIEVEMENTS IN CMB RESEARCH OVER THE LAST DECADE Canadian CMB research obtained international recognition with the pioneering measurement (Gush et al. 1990) of the frequency spectrum by the UBC team using COBRA, a sounding rocket experiment. Though results were just slightly behind the COBE satellite spectrum (Mather et al. 1990), the measurements were a factor of several more precise than COBE’s initial results. The Gush group’s style has been characteristic of the Canadian CMB community: rela- tively small university-based teams involved end-to-end with the experiment, from its initial design conception, its construction, commissioning, deployment, and data analysis. The lion’s share of the work has been accom- Fig. 1.— The Atacama Cosmology Telescope (ACT) under con- plished by graduate students and postdocs who receive struction in Port Coquitllam, B.C. April, 2006. ACT has been “old-school” physics training spanning the full life of recording measurements of small angular scale CMB anisotropies for two years and represents the largest number of TES detectors an experiment, something that has become quite rare on the sky in a single experiment. The readout system warm elec- today. The UBC group continued their program by tronics were built at UBC. building and flying BAM (Tucker et al. 1996) aboard a stratospheric balloon to measure the CMB anisotropy. One of the most exciting results in cosmology to date has been the observation of the acoustic peaks in these experiments. The Atacama Cosmology Telescope the CMB power spectrum by the Boomerang collabora- (ACT) (Fowler et al. 2007) currently has the largest tion (de Bernardis et al. 2000; Netterfield et al. 2001), number of bolometers from a single experiment on the providing a measure of the curvature of space-time and sky and has also announced discoveries of new galaxy demonstrating the universe to be flat within a couple clusters using the SZe. A photo of the ACT telescope, percent, as inflation would suggest it should be. Net- under construction in Port Coquitllam, B.C. is shown in terfield was also involved with an earlier observation of Fig. 1. ACT and APEX-SZ are located at 5000 m on the CMB anisotropy (Netterfield et al. 1996) using the SK Atacama Plateau in Chile, very near to the ALMA site. telescope operating from Saskatoon, Saskatchewan. The Canadians play lead roles in the end-to-end realization Toronto experimental cosmology group evolved from this of all three of these experiments (APEX-SZ, SPT, and heritage. ACT): early design, key technology, data analysis, and Not long after the acoustic peaks were observed, the cosmological interpretation. Cosmic Background Imager (CBI) released its results on The Planck Satellite was launched May 14, 2009 and small angular scale anisotropy (Sievers et al. 2002), es- began routine observations from its L2 orbit in July. tablishing the CITA group’s expertise in the analysis of Planck has finer angular resolution, more bands extend- large CMB data sets and their cosmological interpreta- ing to higher frequency, higher sensitivity, and better po- tion. larization capabilities than WMAP. It has a good shot at The Wilkinson Microwave Anisotropy Probe (WMAP) an early detection of B-mode polarization from inflation- satellite (Bennett et al. 2003) has revolutionized preci- ary gravity waves and will be a focus for Canadian CMB sion cosmology. Halpern has been involved with the ex- data analysis over the next decade. The Canadian effort periment from the onset and key analyses have taken is supported by Canadian Space Agency (CSA). Dick place at CITA. We note that the Canadian WMAP ef- Bond leads the Canadian team for the High Frequency fort has proceeded without any funding from Canadian Instrument (HFI) and Douglas Scott leads the Canadian sources, other than individual NSERC discovery grants. team for the Low Frequency Instrument (LFI). A recent breakthrough for CMB research was the de- EBEX (Granger et al. 2008) and SPI- velopment of technology that allows the construction DER (Crill et al. 2008) are two new CMB polarization of radiometers with thousands of pixels operating near experiments that will be flown aboard NASA strato- the photon noise limit. The increased sensitivity al- spheric balloons. The McGill group is heavily involved lows for precise measurements on small angular scales with EBEX (Figure 2), while Toronto and UBC play and for observation of the faint polarization signature. leading roles with SPIDER. Both experiments rely The Sunyaev-Zel’dovich Effect (SZe) maps of the Bul- heavily on Canadian technology including the attitude let cluster (Halverson et al. 2009) using the APEX-SZ control systems and the multiplexed readout systems. camera (Dobbs et al. 2006) were the first scientific re- Two technological milestones were reached in June sults obtained with a large (300 element) multiplexed 2009 with the test flight of EBEX. This marked the array of bolometers. The same technology was used first demonstration in a space-like environment of (1) a by the South Pole Telescope (Carlstrom et al. 2009) large array of Transition Edge Sensor (TES) bolometers for the first discovery of previously unknown clus- and (2) a SQUID-based multiplexed readout system. ters using the SZe (Staniszewski et al. 2009). The These are enabling technologies for a future CMB third and newest node of experimental cosmology in polarization satellite, which is the long term goal for Canada, the McGill group, evolved from Dobbs’ con- much of international community. tribution developing the multiplexed readout system for In addition to the CMB experiments described above, Canadian CMB researchers have used their expertise LRP Whitepaper: Experimental CMB 3

Fig. 3.— The McGill digital frequency domain multiplexer (one FPGA circuit board shown left, and a full crate of electronics providing readout for 1000 channels shown right) is being deployed on EBEX, POLARBEAR, ASTE continuum camera, and the South Pole Telescope polarimeter. It was test flown in June 2009 on the EBEX balloon polarimeter marking the first demonstration in a space-like environment of (1) a large array of Transition Edge Sen- Fig. 2.— The EBEX balloon-borne polarimeter was launched by sor (TES) bolometers and (2) a SQUID-based multiplexed readout NASA for a test flight in June 2009, marking the first operation system. These are enabling technologies for a future CMB polariza- of a large array of transition edge sensor bolometers and a multi- tion satellite, which is the long term goal for much of international plexed SQUID readout system in a space-like environment. The community. experiment is scheduled for its science flight in 2011. The readout system was designed and built at McGill university.

system (TDM) (Battistelli et al. 2008) (figure 4). The with CMB-related technology and experience with low TDM is in use (or scheduled for use) on ACT, ABS, Bi- signal to noise sky signals to enable experiments at other cep2, Clover, Keck, Poincare, Piper, Poincare, Spider, wavelengths. These include the SCUBA2 project, which all funded CMB experiments. In addition, TDM is be- uses the UBC readout electronics and the BLAST sub- ing used for other wavelengths, including the SCUBA2, mm balloon borne telescope (Pascale et al. 2009), which Z-specII, and Goddard Sofia cameras. The McGill group recently released striking maps of the sub-mm universe has developed a digital frequency domain multiplexer (e.g. (Devlin et al. 2009)). These are just a few of many (DfMUX) (Dobbs et al. 2008) (figure 3) which builds on examples. the heritage of the analog frequency domain multiplexer that Dobbs co-developed while at Berkeley. The DfMUX 3. UNIQUE CANADIAN CAPABILITIES system uses NIST SQUIDs for cold pre-amplification. The Canadian CMB research community has devel- The frequency domain system is in use or scheduled for oped many unique capabilities for which it has a role of deployment on APEX-SZ and South Pole Telescope (ana- international leadership. These include the following. log version) and EBEX, POLARBEAR, South Pole Tele- Multiplexed Readout Electronics for Bolometers: scope Polarimeter, and ASTE (digital version). Presently the vast majority of CMB experiments have Canadians have established international leadership in chosen Transition Edge Sensor (TES) bolometers for de- technology for TES detector multiplexing, which will tection of mm-wave light, consistent with expectations likely earn us an invitation to participate in a future from the U.S. Task Force for Cosmic Microwave Back- CMB polarization satellite. ground Research (Weiss et al. 2005), that recommended Electronics for Pointing and Control of Balloon Payloads: “technology development leading to receivers that con- The Toronto instrumentation team, led by Netter- tain a thousand or more polarization sensitive detectors, field, have established themselves as world leaders in sys- and adequate support for the facilities that produce these tems for control of pointed balloon payloads. They are detectors.” The recommendation also identified that “It presently operating out of a high-bay facility purpose- is important to keep open a variety of approaches until built for developing balloon payloads. Their atti- a clear technological winner has emerged. Nevertheless, tude control system (ACS) was originally developed for highest priority needs to be given to the development of BLAST. It will be used for BLASTpol and SPIDER and bolometer-based polarization sensitive receivers.” TES forms the foundation of the EBEX mission ACS. This is detectors allow for lithographic construction of kilo-pixel a well designed, modular system that will undoubtedly arrays that approach the photon statistics limit. For ar- be adopted by other missions as well. rays of a thousand or more TES detectors, constraints on Online Monitoring Software: complexity and heat load make it necessary to multiplex KST is an online monitoring software system, orig- the signal from many bolometers at the cold stage into a inally conceived in Netterfield’s group and now be- smaller number of output channels–multiplexed readout ing developed and maintained by both the UBC and is key technology in enabling large TES arrays. Toronto/CITA groups. It allows scientists to accurately Almost all TES bolometers that are presently observ- monitor timestreams and derived data products from ing the sky or scheduled to deploy in the upcoming timestreams in realtime. decade are read out using multiplexed technology co- The KST effort receives funding from the CSA through developed by Canadian institutions. The UBC group, its Planck programs and is used by the international in partnership with NIST who provides the supercon- Planck collaboration for trend analysis and instrument ducting cold components, has developed the room tem- monitoring. Beyond Planck, it is used by just about ev- perature electronics for the time domain multiplexed ery CMB research group on the planet. 4

Analysis Software and Algorithms: Canadian researchers have been key players for the analysis of CMB data and its cosmological interpretation. This leadership role has been exercised for both experiments that have Canadian hardware contributions and those that do not, such as CBI, ACBAR, and Planck. The computing resources at CITA have helped to enable this role. Foregrounds: The HIA-DRAO has aided CMB research by carrying out the Planck Deep Field Survey to measure foreground emission and by offering a suitable dish and an excellent northern site for a 10 GHz all sky mapping proposal. DRAO’s mandate for supporting University initiatives is crucial to these efforts. 4. HIGHLY QUALIFIED PERSONNEL The experimental CMB field provides second-to-none training opportunities for students and postdocs. The timeframe of a typical CMB experiment provides a unique opportunity and skill-set to students and postdocs. Within the span of a typical PhD they have the opportunity to participate in the end-to-end development, commissioning, and analysis of a cutting edge scientific Fig. 4.— The UBC Multi Channel Electronics are shown on the instrument. This training environment is one of the most BICEP2 experiment at the South Pole in Feb 2010. This system will be flown on the SPIDER balloon borne polarimeter in 2011 and important contributions of the lab to Canadian academia is deployed or being deployed on the ACT, ABS, Bicep2, Clover, and industry. Keck, Poincare, Piper, Poincare, Spider, SCUBA2, Z-specII, and The groups involved with instrumentation (Dobbs, Goddard Sofia instruments. Halpern, Netterfield) are presently providing training and field experience to five postdocs, six professional staff (primarily young engineers), eleven graduate students, and a half dozen undergraduates. We stress the uniqueness of the training these individuals receive and emphasize the importance of their future contributions to the Canadian academic, industrial, and even military scenes. In addition there are several postdocs, research associates, graduates students and undergrads working on data analysis projects with the theory-background members of the community (Bond, Holder, Scott). 5. CANADIAN OPPORTUNITIES In the first half of the upcoming decade, the CMB community will be focusing on deployment, operation, and analysis of the new generation of instruments it has co- pioneered with its international partners. These include (with senior researcher having primary involvement in parentheses): • The South Pole Telescope and its polarimeter upgrade (Dobbs and Holder) SPT will continue its small angular scale temperature anisotropy survey until roughly 2012, when it will be upgraded with a polarization sensitive camera using McGill readout electronics. • The Atacama Cosmology Telescope (Bond and Halpern) ACT is releasing its first science results now and plans to upgrade its camera for polarization sensi- Fig. 5.— The BLASTpol gondola is shown in the University of tivity within a couple years. Toronto high-bay, a facility constructed specifically for the development of balloon payloads. The BLAST attitude control system • KECK Telescope (Halpern) is being used for BLAST, EBEX, BLASTpol, and SPIDER. It will The BICEP telescope at the South Pole has evolved likely be used for others as well. into a much more ambitious large angular scale polarimeter called Keck. UBC has provided the warm readout electronics. LRP Whitepaper: Experimental CMB 5

• POLARBEAR (Dobbs) use Baryon Acoustic Oscillations (BAO) to follow Polarbear is a ground-based experiment being sited the CMB’s acoustic peaks down to the lower red- on the Chilean Atacama plateau. McGill provides shifts (z=1-3) where dark energy ‘turns on’. The the readout. experiment would be hosted at the DRAO in Pen- ticton and constructed of digital fast Fourier trans- • EBEX (Dobbs) form telescopes. The cost and timeframe will be of The EBEX balloon-borne polarimeter had its test the same scale as the ground-based CMB experi- flight in June 2009 and is scheduled for a science ments discussed here. Refer to the 21 cm whitepa- flight in 2011. per for more info. • SPIDER (Bond, Halpern, Netterfield) 6. CHALLENGES FOR CMB RESEARCH IN SPIDER, which uses the UBC readout electronics CANADA and Toronto ACS, will be test flown in 2010/11 and Despite the achievements of the community to date, is scheduled for a science flight a year later. there remains significant challenges for this research in • Plank LFI and HFI (Bond and Scott) Canada. While the Planck mission duration is just 2 years, The CMB community has achieved internationally ac- data analysis will continue for several years after- claimed results and developed world leading technology, wards. sometimes with little or no Canadian funding. Though Canadians have been at the forefront of some of the most we have listed only those projects that have end-to- important measurements of the CMB (often as first au- end Canadian involvement and have not included exper- thors in large international projects), PI-level leadership iments to which Canadians are providing technology but of these projects has justifiably remained in the coun- have chosen not to have a comprehensive role in. try providing the lion’s share of funding - this has not Beyond mm-wavelengths, the Toronto group is recently been in Canada. The community aspires for a presently heavily involved with building a polarization Canadian led project in the upcoming decade to allow sensitive sub-mm balloon-borne instrument, BLASTpol, Canadian researchers to fulfill their potential. In part and the UBC group has built the readout electronics for because the CMB community is small in comparison to SCUBA2 and is commissioning the instrument. the larger Canadian astronomy community, funding for Looking forward to new experiments, the community is such a project may be difficult. This is particularly true primarily eyeing CMB polarization science for the latter for ground-based opportunities where the CSA is not in- half of the upcoming decade: volved. It is noted that many of today’s most prominent CMB • The international CMB community has converged experiments with Canadian partnership (e.g. WMAP, towards the goal of a dedicated CMB polariza- ACT, SPT, POLARBEAR, Ebex, and Spider) have gone tion satellite, commonly referred to as CMBpol in forward without a presence in the previous LRP, and in- the U.S.A. The European effort has gone by the deed the majority of these were not yet on the horizon. name BPOL. The Japanese community is studying The timescale for these experiments is often faster than a small-sat polarimeter called LITEBIRD. ASP is the LRP lifecyle. This means that one of the most im- a midex proposal for a polarizing FT spectrometer portant challenges for the CMB community is being able acting as a low ` polarization probe. Whichever to respond to opportunities and realize new ideas on a effort gains traction, Canadians will likely be in- short time scale. The community has demonstrated it volved with readout electronics, software, and data can do this in the format of international partnerships. analysis. It is feasible to consider a mid-sized Cana- It is important that funding vehicles exist that will allow dian led mission as well–Canada has the necessary Canadians to realize their potential, both in partnerships expertise. (as has been the case) and Canadian-led enterprises. CFI and CSA are likely to be important in this regard. • A follow-up to CSA supported balloon missions The importance of the Canadian Space Agency (CSA) EBEX and Spider. This mission would combine for CMB research across Canada cannot be overstated. the best of these two pathfinder missions and be a It funds balloon missions such as EBEX and SPIDER pre-cursor to a large international satellite mission, and the Planck satellite. It is anticipated that the exper- described above. tise developed in Canada will continue to make Cana- At the same time, the community is continuing its tra- dians welcome partners for a future CMB polarization dition of leveraging its experience within the CMB field satellite, presumably for the development of the detec- to increase research opportunities in other related fields tor readout system and software/analysis, which needs of astronomy. This has been the case for sub-mm astron- continued support from CSA. It is equally important omy with BLAST and SCUBA2. Looking forward: that CSA fund the science return for its space astronomy missions at an adequate level. CSA should endeavor to • Netterfield is studying new opportunities to apply fund science analyses through the lifetime of the mission, his group’s ballooning expertise to a balloon-born probably using an SSEP (grant not contract) vehicle, so wide field optical imager instrument. long as these missions continue to hold promise for excellent scientific results and excellent scientists are signed • Bond, Dobbs and Halpern have been studying and on to the projects. designing a pan-Canadian 21 cm hydrogen inten- The CSA’s move towards expanded infrastructure and sity mapping experiment called CHIME that will funding support for small missions (such as balloon pay- 6

loads and nano-sats) is welcome to the CMB ﬁeld and nology. extremely important for capacity building in space tech-

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